The Dems need 9 seats in the Senate to achieve a filibuster-proof majority.

Two seats are already in their pocket: Warner in Virginia and Udall in New Mexico.

They are running significant leads in two other races: Udall in Colorado and Shaheen in New Hampshire.

They are trending toward slight leads in three races: Merkley in Oregon, Hagan in North Carolina, and Begich in Alaska.

The race between Coleman (R) and Franken (D) in Minnesota is likely neck and neck (with very erratic polling).

Recent polls have them closing on some normally-safe Republicans in Red states like Mississippi, Texas, Kentucky, and Georgia.

Chuck Schumer, head of the Democratic Senatorial Campaign Committee believes that Collins' (R) seat in Maine is vulnerable too, though that is unlikely.  See pollster.com for an interactive map on these races.

With this breakdown in mind, Democrats will likely pick up at least 4 seats.  I would guess that it is more likely they'll take 6-8. I'll currently predict 6, giving them OR and NC while keeping AK (Stevens is coming back) and MN (Franken is repulsive).  If Obama has a very strong night, however, expect him to take those 8 and possibly assist in an upset of one or two others (perhaps Georgia and MS) for a total of 9-10.  That was be a sweeping victory for Dems to say the least.

Remember, the magic number is 9.  Even with such a victory, mustering an entire party's vote against a filibuster would be quite difficult since some Dems represent Red states, but not out of the question.